1. Field of the Invention
The present invention relates to a collimator lens of graded index type to collimate radiated rays from a semiconductor laser into parallel rays in a light beam scan apparatus such as an optical memory, an optical readout device and a laser beam printer.
2. Description of the Prior Art
In the collimator lens for the above-mentioned apparatuses, it is essential for collimation of radiated rays from a semiconductor laser at low aberration that the residual wavefront aberration of the lens is within .lambda./4, where .lambda. is the osillation wavelength of the semiconductor laser and generally in a range of 760 nm.ltoreq..lambda..ltoreq.850 nm. Also the collimator lens should be in small size, light weight and low cost.
In such light beam scan apparatuses, a single wavelength is usually used. Consequently, if the transmittance of the lens is as high as possible, the light emission power of the laser can be correspondingly suppressed and the reliability of the apparatus be improved. It is favorable for the measures that an antireflection coating is applied to the lens surface and the number of lenses is made as small as possible.
As a collimator lens to satisfy the above-mentioned requirements, one constituted by combination of three or four usual spheric lenses each having the uniform refractive index is known. Since such a usual collimator lens has as many as six to eight machining surfaces for spherical polishing and the lens diameter is in general very small, however, the polishing is difficult and high technical skill is required for combination of lenses such as cementing process, resulting in high cost.